Lanthanide-based catalyst systems are known to be useful for polymerizing conjugated diene monomers to form polydienes having high cis-1,4-linkage contents, low 1,2-linkages, narrow molecular weight distributions, and linear backbones. These cis-1,4-polydienes containing a linear backbone are believed to provide better tensile properties, higher abrasion resistance, lower hysteresis, and better fatigue resistance as compared to the cis-1,4-polydienes prepared with other catalyst systems such as titanium-, cobalt-, and nickel-based catalyst systems. Therefore, the cis-1,4-polydienes made with lanthanide-based catalysts are particularly suitable for use in tire components such as sidewalls and treads.
However, one disadvantage of the cis-1,4-polydienes prepared with lanthanide-based catalysts is the relatively high cost of the lanthanide compounds. Furthermore, high catalyst loadings of the lanthanide catalysts and high loadings of Al co-catalysts are often needed to obtain polymers with good processability. A common method to lower the amount of lanthanide catalyst is to use an aluminoxane co-catalyst. Still, the aluminoxane co-catalysts are also expensive and typically a large excess is required.
Therefore, there is a need to develop a process for producing lanthanide-based cis-1,4-polydienes with low catalyst loadings and/or low Al co-catalyst loadings to maintain the combination of high cis-1,4 content, low 1,2 content, and a linear backbone while minimizing the cost of the catalyst components.